Generally, there are four classes of pollutant that are legislated against by inter-governmental organisations throughout the world: carbon monoxide (CO), unburned hydrocarbons (HCs), oxides of nitrogen (NOx) and particulate matter (PM). As emissions standards for permissible emission of pollutants in exhaust gases from vehicular engines become progressively tightened, there is a need to provide improved catalysts that are able to meet these standards and which are cost-effective.
For compression ignition engines, such as diesel engines, a catalysed soot filter (CSF) is typically used to treat the exhaust gas produced by such engines. CSFs generally catalyse the oxidation of (1) carbon monoxide (CO) to carbon dioxide (CO2), (2) HCs to carbon dioxide (CO2) and water (H2O) and (3) the oxidation of PM filtered from the exhaust gas. The two most important PM oxidation reactions are oxidation in nitrogen dioxide (NO2+C→NO+CO) and oxygen (O2+2C→2CO or O2+C→CO2). Sources of NO2 for the former reaction are the engine itself and nitrogen monoxide (also present in the exhaust gas) oxidised either on an upstream substrate monolith comprising a diesel oxidation catalyst (DOC) or on the filter catalyst itself. Exhaust gas temperatures for compression ignition engines, such as diesel engines particularly for light-duty diesel vehicles, are relatively low (e.g. about 400° C.) and so one challenge is to develop durable CSF catalyst formulations with low “light-off” temperatures.
The activity of oxidation catalysts, such as CSFs and DOCs, is often measured in terms of their “light-off” temperature, which is the temperature at which the catalyst starts to perform a particular catalytic reaction or performs that reaction to a certain level. Normally, “light-off” temperatures are given in terms of a specific level of conversion of a reactant, such as conversion of carbon monoxide. Thus, a T50 temperature is often quoted as a “light-off” temperature because it represents the lowest temperature at which a catalyst catalyses the conversion of a reactant at 50% efficiency.
Low Emission Zones (LEZs) are areas or roads across Europe, e.g. Berlin, London, Stockholm, Eindhoven etc., where the most polluting vehicles are restricted from entering (see http://www.lowermissionzones.eu/what-are-lezs?showall=1&limitstart=). There is growing evidence that poor air quality is bad for health and life expectancy. Nitrogen dioxide is considered to have both short-term and long-term effects on health. It affects lung function and exposure enhances the response to allergens in sensitised individuals. It has been suggested that apparent effects of nitrogen dioxide on health may be due to particles or to its combination with particles. NO2 can also contribute to reactions causing photochemical smog. EU Air Quality Standards (binding on EU Member States) sets Limit Values for the protection of human health. The EU Air Quality Standard for inter alia NO2 was set from 1 Jan. 2010 at 200 μg/m3 (105 ppb) average over a 1 hour period, not to be exceeded >18 times a calendar year; and a 40 μg/m3 (21 ppb) average per calendar year.
There is therefore a need in the art for exhaust systems which avoid or reduce NO2 emission into the atmosphere, particularly for vehicles accessing LEZs. These can include factory fit exhaust systems and systems to be retrofitted to existing vehicles.
WO 00/34632 discloses a system for treating the exhaust gases from diesel engines comprising a first catalyst effective to oxidise hydrocarbons, a second catalyst effective to convert NO to NO2, a trap for particulates, on which particulates may be combusted in NO2. The first catalyst can be platinum dispersed on ceria or on a metal oxide washcoat which incorporates ceria. The Examples explain that: “It is evident that once the HC (represented by C3H6) has been removed in the first oxidation step the oxidation of NO to NO2 can take place more completely”.
Catalysts that are used to oxidise carbon monoxide (CO), hydrocarbons (HCs) and sometimes also oxides of nitrogen (NOx) in an exhaust gas emitted from a compression ignition engine generally comprise at least one platinum group metal, such as platinum or palladium. Platinum is more active than palladium at catalysing the oxidation of CO and HCs in the exhaust gas from a compression ignition engine and the inclusion of palladium in such catalysts was generally avoided because of its susceptibility to poisoning by sulphur. However, the use of ultra-low sulphur fuels, the relative cost of palladium to platinum, and improvements in catalyst durability that can be obtained by inclusion of palladium have resulted in catalyst formulations comprising palladium, especially formulations comprising both palladium and platinum, becoming favoured.
Even though, in general, the cost of palladium has historically been lower than that of platinum, both palladium and platinum are expensive metals. Oxidation catalysts that show improved catalytic activity without increasing the total amount of platinum and palladium, or that show similar catalytic activity to existing oxidation catalysts with a lower amount of platinum and palladium, are desirable.